Planar patch antennas for RF (radio frequency) reception and/or transmission are becoming increasingly popular because of their small size and other useful attributes. However, they do have some drawbacks, such as relatively narrow bandwidth. Hence, techniques have been and continue to be developed to increase the bandwidth of such antennas. For instance, multiple patches of different sizes layered together can increase bandwidth. More recently, the use of an L-shaped probe instead of a conventional strip line or microstrip feed mechanism has been used to increase the bandwidth of planar patch antennas. H. Wong, L. Lau, and K. Luk, “The design of dual-polarized L-probe patch antenna arrays with high isolation”, IEEE transactions on antennas and propagation, volume 52, number 1, January 2004. This reference discusses a dual polarization antenna utilizing two L-shaped probes oriented orthogonally to each other in order to feed a single patch. The authors claim that a 20% or greater bandwidth can be obtained with this design.
However, the use of two orthogonal L-probes suffers from at least two significant deficiencies. First, it has a poor isolation between the two ports (i.e., between the two polarizations). That is, there can be significant coupling between the two ports such that signal on the first feed line of the first polarization pollutes the signal of the other polarization on the other feed line. Second, it has poor cross polarization properties. The isolation and cross-polarization levels could be as high as −10 dB. Typically, for good performance of radars, the isolation and cross-polarization levels should be on the order of −20 dB. Specifically, when two L-probes (or any other feed mechanisms, for that matter) are oriented orthogonally to each other, ideally, there should be no cross polarization between the two probes. Particularly, the E field of each probe should be parallel to the probe and, therefore, the E field of one probe should have no effective field strength at the other probe because the other probe is orthogonal thereto. However, in practice, this has proven to be far from true.
In the aforementioned paper, Wong et al. propose one solution to help increase isolation involving the use of the balanced L-probes. Id. According to this solution, instead of using a single L-probe per polarization, two L-probes oriented in opposing directions and fed with signals 180° phase shifted relative to each other are used to feed each polarization. The feed network is rather complex in order to feed each of the two L-probes associated with each polarization with the same basic signal, but 180° out of phase there with. This is achieved by branching the feed line into two lines, one of the branches being a half wavelength longer than the other branch.
This design has been found to provide substantial benefits in terms of increased isolation and, often, decreased cross-polarization. But the major disadvantage is that it requires a very complex feed network in the feed network layer of the planar antenna. Furthermore, when the feed network is microstrip, there is distortion in the antenna radiation patterns and increased cross-polarization levels.
A complex feed network is extremely disadvantageous, particularly in antenna arrays, because there often is a need or desire to place additional circuitry in this layer, such as RF transmission lines, DC lines, control lines, etc. Specifically, these lines often need to be placed in the same layer as the feed network between two ground planes in order to isolate the signals on those lines from the radiating (or receiving) patches of the antenna.
It also is known in the prior art to use disc coupling, instead of L-probe coupling. In these types of systems, instead of using an L-shaped probe, the feed network is coupled to one or more disc shape probes that capacitively couple to the patches.